Had a handful of these over the past few days. How is it even possible if the number is blocked?

Could it be that it's blocked on the phone only? If the voicemail is being handled by the network how would they know you've blocked the number on your phone. So the blocked number would be ignored by your phone and not ring, then the network will take over and direct it to voicemail like other unanswered calls.

Ah yes, of course. And if the automated spam-dialler does not hang up before the voicemail message beep ...

Annoying.